In San Francisco last week, I met up with an old Facebook source, and we got to talking about who the social network's biggest ad clients are.

There is a list of them at the bottom of this story.

Facebook's biggest ad clients are one of its most closely guarded secrets. The company makes about $10 billion a year in revenue, almost all of which comes from advertising. But it never talks about who its biggest spenders are.

For the past couple of years, Business Insider has used its sources to come up with what we believe is a list of Facebook's biggest ad clients. Last year, it was probably Samsung.

While Samsung may be Facebook's single biggest marketing client, it's probably not Facebook's biggest media-buying client. That's because Samsung's adspend — like everyone else's — is delivered through a media buyer. Facebook has an ecosystem of media buyers that bundle big corporate ad budgets together. They're called Preferred Marketing Developers.

Anyway. This, according to my source — who has been inside many a PMD meeting at the company — is who Facebook's biggest paid media clients are right now:

The list is not ranked in order of size. We previously heard there were as many as 300 PMDs funneling money, one way or another, into Facebook. But last week a different source inside Facebook said he believed the total number of PMDs was now "more than 200." That would imply some consolidation has taken place – which Business Insider predicted would happen last year.

The five biggest individual brands advertising on Facebook, as we reported last year, are likely to be: